I'm sure you found out by now what it does but just incase you didn't, your light_environment is basically your "sunlight". Any skybox textured brush will cast a light at your given pitch/yaw (which needs to be at around -45 to -90 pitch to visibly cast downwards and light your map, otherwise it will just cast light sideways). Thus, lighting your outside areas.
You can also control the standard level of darkness of your shadows, but this is a very subtle use that can go horribly wrong if you don't take care.
you'll want to match your your light_environment colour with your skybox's ambience for maximum immersion. I couldn't find the skybox library here on TF2maps.net but did find them on FPSB, but to save your soul from setting foot on the internets anus i'll copy and paste the variables here:
sky_badlands_01
Sun Angle: 0 210 0
Sun Pitch: -30
Brightness: 251 201 155 250
Ambience: 153 159 187 170
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_dustbowl_01
Sun Angle: 0 30 0
Sun Pitch: -25
Brightness: 254 210 152 800
Ambience: 151 135 180 420
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_goldrush_01
Sun Angle: 0 108 0
Sun Pitch: -35
Brightness: 254 226 186 600
Ambience: 118 114 139 350
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_granary_01
Sun Angle: 0 130 0
Sun Pitch: -15
Brightness: 255 226 196 500
Ambience: 116 113 149 300
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_gravel_01
Sun Angle: 0 167 0
Sun Pitch: -32
Brightness: 250 216 146 850
Ambience: 106 134 174 500
HDR Brightness: 250 216 149 650
HDR Ambience: 106 134 174 380
sky_hydro_01
Sun Angle: 0 51 0
Sun Pitch: -37
Brightness: 244 215 193 750
Ambience: 156 178 255 250
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_night_01
Sun Angle: 0 108 0
Sun Pitch: -35
Brightness: 66 73 100 175
Ambience: 66 73 100 175
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_tf2_04
Sun Angle: 0 145 0
Sun Pitch: -45
Brightness: 216 207 194 700
Ambience: 200 202 230 60
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_trainyard_01
Sun Angle: 0 300 0
Sun Pitch: -65
Brightness: 253 243 208 600
Ambience: 124 138 203 500
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
sky_well_01
Sun Angle: 0 300 0
Sun Pitch: -35
Brightness: 248 197 139 400
Ambience: 142 154 182 200
HDR Brightness: -1 -1 -1 1
HDR Ambience: -1 -1 -1 1
edit: Found extra info on the VDC:
http://developer.valvesoftware.com/wiki/Sky_List#team_fortress_2_materials.gcf